This invention relates to arrangements, systems or receivers using monopulse techniques, such as those receivers used for radar surveillance or for radio frequency (RF) missile seekers, and more particularly to improved arrangements for locating targets, including up to two targets within the main beam of the antenna.
A monopulse antenna system includes plural antenna elements which receive the signal whose location is to be identified, and also includes various couplers which add the signals from various combinations of the antenna elements for generating sum signals, and azimuth and elevation difference signals. One type of prior-art monopulse antenna has four feed horns at the focus of a reflector, and a monopulse array antenna may have many antenna elements, beamformed to generate the desired sum and difference signals. In the context of such antennas, the term xe2x80x9cazimuthxe2x80x9d and xe2x80x9celevationxe2x80x9d are conventionally used, but refer to two mutually orthogonal measurements rather than to actual orientations.
In a monopulse antenna system, the presence of the target is determined by the existence of a signal within the sum beam. In the presence of a target as determined from the sum beam, the elevation difference signal is divided by the sum signal to generate a value which establishes the elevation angle, and the azimuth difference signal is divided by the sum signal which determines the azimuth angle. The quotients of the divisions are applied to look-up tables in order to determine the corresponding angular location within an antenna beam.
The beamwidth of an antenna is inversely related to the dimensions of an antenna measured in wavelengths; as the antenna gets smaller relative to the wavelength, the beamwidth gets larger. Some systems, like radar systems or RF missile seeking systems, detect and track their targets by use of the mainlobe of an antenna. In general, mobile devices must use small antennas, even when operated at the highest practical frequency, and the antenna thus tends to have a wide main beam, which imposes limits on the ability of a system to identify closely spaced sources, which in the case of a missile might cause the missile to home on a decoy located near the actual target, or to home on ground reflections.
In the context of a ground-based search radar system, the time required to complete the volume scanning requires that the antenna beam be relatively broad or large. Moreover, advanced search radar favors low frequency operation for low radar cross section (RCS) target detection advantage. Thus, the antenna beam of the scanning radar antenna, being broad, is likely to contain a plurality of targets. The look-up tables of a monopulse antenna system cannot provide angles in the presence of multiple targets within the main beam of the antenna. Improved monopulse target or source location is desired.
For instance, advanced air defense missile seekers have been developed. The requirement is that the seeker has capability to track incoming tactical ballistic missile (TBM), cruise missile and fighters and to home on these targets. However, angle deception techniques such as towed decoy and ground bounce jamming have emerged as real threats for denying missile tracking and target homing. Also, ground based radar have been designed which are susceptible to have multiple targets within the main beam. In addition, advanced synthetic aperture radar (SAR) systems have been developed which face adverse electronics-counter-measure (ECM) threats including mainbeam deceptive jammers. Next generation shipboard fire-control radar, which need to operate in adverse mainlobe jamming environment have been developed.
Prior work for multiple target angle estimation within the mainbeam include extension of monopulse technique and modern subspace eigenstructure analysis. Monopulse processing techniques for multiple targets are discussed in xe2x80x9cMultiple Target Monopulse Processing Techniques,xe2x80x9d by Peebles and Berkowitz, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-4, No. 6, November 1968. However, the technique disclosed therein requires special antenna configurations that are much more complicated than the sum-difference channels normally used in monopulse radars. Moreover, the proposed technique generally requires six beams to resolve two targets. The article xe2x80x9cComplex Indicated Angles Applied to Unresolved Radar Targets and Multipath,xe2x80x9d by Sherman, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-7, No. 1, January 1971, concludes that with a conventional monopulse configuration, a single pulse solution is impossible. This reference discloses a method to resolve two targets using two independent measurements, but it is not a xe2x80x9cmonopulsexe2x80x9d technique per se.
Modern high-resolution signal subspace algorithms such as MUSIC, root-MUSIC, minimum-norm algorithms and others overcome the beamwidth limitation by exploiting measurements over multiple channels with a multiple number of snapshots. Essentially, these algorithms make use of the eigenstructure of the covariance matrix of sensor outputs to estimate the number of signal sources and the direction-of-arrival (DOA) of the sources. These techniques exhibit a high-resolution capability in that they offer a practical means of separating them in less than the Rayleigh resolution limit determined by the antenna aperture size. However, conventional super-resolution algorithms such as MUSIC are computational intensive as it requires two-dimensional manifold search. Moreover, these methods require multiple snapshots for covariance matrix estimation.
Several references disclose recently developed super-resolution techniques for resolving multiple sources impinging planar antenna array based on two-dimensional root-finding method such as PRIME-MUSIC and invariance principle such as ESPRIT. These references include: U.S. patent application Ser. No. 09/128,282 for xe2x80x9cMonopulse System For Target Locationxe2x80x9d by K. B. Yu; xe2x80x9cStructured null space problem,xe2x80x9d SPIE conference on Advanced Signal Processing Algorithms, Architectures, and Implementations VIII, Jul. 22-24, 1998, San Diego, Calif. Vol. 3461, pp. 280-285, by F. T. Luk and K. B. Yu; xe2x80x9cA Class of Polynomial Rooting Algorithms for Joint Azimuth/Elevation Estimation Using Multidimensional Arrays,xe2x80x9d in 28th Asilomar Conference on Signals, Systems and Computers, Pacific Grove, Calif. 1994, by G. F. Hatke and K. W. Forsythe; and xe2x80x9cESPRITxe2x80x94Estimation of Signal Parameters Via Rotational Invariant Techniques,xe2x80x9d IEEE Transactions of Acoustics, Speech, Signed Processing, Vol. 37, pp. 984-995, July 1989, by R. Roy and T. Kailath. These techniques make use of multiple snapshots for covariance matrix accumulation and may suffer from the target fluctuations between pulses. Also, there may not be time for multiple measurements especially when pulse compression is used to generate fine range-doppler profile. A single snapshot technique based on four monopulse channels is disclosed in copending patent application Ser. No. 09/607146 for xe2x80x9cMonopulse Radar Processor For Resolving Two Sources,xe2x80x9d filed Jun. 29, 2000 by Y. Zheng and K. B. Yu. This technique is based on measurement modeling and the algorithm involves a quadratic equation followed by a linear equation.
An object of this invention is to provide a method and system for identifying the location or angular direction of a single target within the main beam of a monopulse antenna.
Another object of the present invention is to use a matrix monopulse ratio processing technique to identify the location or angular direction of a single target within the main beam of a monopulse antenna.
These and other objectives are attained with a method and system for identifying the locations of plural targets lying within a main beam of a monopulse antenna including four ports for generating sum, elevation difference, azimuth difference and double difference signals. The method comprises the step of forming a monopulse ratio matrix from the sum, elevation difference, azimuth difference and double difference signals. Eigenvalues of the monopulse ratio matrix are determined, and values of the eigenvalues are used to determine the angular locations of the plural targets. Preferably, the eigenvectors are determined by performing an eigenvalue decomposition of the complex monopulse ratio matrix to generate complex eigenvalues and the azimuth and elevation angle of the target can be determined from the real and imaginary part of the eigenvalue by the use of a look-up table.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.